Geochronological Potential of Isoleucine Epimerization in Calcite Speleothems

The extent of isoleucine epimerization in a calcite speleothem was determined to evaluate the amino acid racemization method in abiotic calcite. A 5.5-cm-thick flowstone slab from Hamarnesgrotta, northern Norway, was analyzed for amino acid concentration, composition, and isoleucine epimerization at 26 levels through the sequence. U-series dates provide an independent chronologic control. Epimerization increases monotonically with stratigraphic depth and linearily with U-series age, independent of amino acid concentrations. The rate of epimerization is calibrated against the U-series dates, and extrapolation into lower strata beyond the U-series limit yields absolute age estimates that are consistent with paleomagnetic data from the same speleothem. The results suggest that, if adequately calibrated, amino acid dating is applicable to speleothem material reaching time spans beyond the range of conventional dating methods. Amino acids in the speleothem were probably derived from surface soils and are associated with brown humic stains in the calcite.